In mobile communications of a CDMA scheme and digital terrestrial television broadcast scheduled to be implemented in the immediate future in Japan, a so-called multicarrier communication scheme is used wherein information is transmitted using a plurality of carriers distributed over a wideband.
Providing the same number of high frequency power amplifiers as the number of carriers in multicarrier transmission would be a great burden in terms of the size and cost of the apparatus.
Therefore, a multicarrier signal needs to be collectively amplified by one high frequency power amplifier.
When a high frequency signal of a wide band is amplified by one high frequency power amplifier, various distortions due to the non-linearity of the high frequency power amplifier (non-linear distortions: for example, a mutual modulation distortion) easily appear.
Methods for reducing non-linear distortions are mainly classified into a pre-distortion scheme and a feed forward scheme.
The pre-distortion scheme is a scheme in which a pseudo-distortion for canceling out distortion expected to occur in a power amplifier is preliminarily given to a signal to be input into the power amplifier.
The feed forward scheme is a scheme that extracts a distortion component contained in the output signal of a power amplifier, inverts the phase of the distortion component, and feeds back the inverted signal to the output signal of the power amplifier to cancel out the distortion component.
In recent years, a scheme has been proposed that combines these typical distortion compensation techniques. An analog distortion compensation circuit having an adaptive pre-distortion circuit and a feed forward power amplifier combined is disclosed in U.S. Pat. No. 5,760,646.
As can be seen from the circuit diagram of FIG. 1 of the above publication, the distortion compensation circuit disclosed in this U.S. Pat. No. 5,760,646 is configured such that a distortion component signal (an error signal fed back to a main path after inverted in phase) that appears in the feed forward loop of a typical feed forward power amplifier is also input into a pre-distortion function generator (a circuit generating a control signal for a pre-distortion characteristic) to control an analog pre-distortion circuit to perform adaptive pre-distortion processing.
The distortion compensation circuit disclosed in the above U.S. Pat. No. 5,760,646 detects distortion contained in the output signal of the power amplifier when being a high frequency analog signal (a RF analog signal), and performs RF pre-distortion processing (pre-distortion processing on the high frequency analog signal) based on the distortion component signal.
However, there is a limit to accuracy in detecting the distortion component signal (RF error signal) from the high frequency analog signal (limit due to processing of detecting the analog signal).
Also there is a limit to accuracy in pre-distortion processing using an analog circuit due to analog signal processing.
Therefore, the capability of this distortion compensation circuit to compensate for non-linear distortions is not so high.
Meanwhile, with respect to multicarrier transmission in mobile communications of the CDMA scheme and digital terrestrial television broadcast scheduled to be implemented in the immediate future in Japan, it is required to satisfy unprecedented strict standards in order to maintain high communication quality.
That is, it is required to compensate for distortion of the transmit signal having an extremely wide band with high accuracy and stably adapting to the environment.
The prior art technology cannot meet such strict requirements.
Moreover, in the mobile communications field, much smaller transceivers with reduced power consumption and cost to an extreme extent are required. This makes it further difficult to realize a great improvement of the capability to reduce non-linear distortions of a wideband.